Wishing they would all be nice, just does not work with roosters. And the plan you have, is not working for the set up you have. Set ups tend to be rather permanent, and expensive to change. If you can't change the set up, then the only thing you can change is the flock.
If you absolutely want to keep these roosters, then cull the hen. She is old, and probably will not live a lot longer. It would be better to give her the coop de grace, rather than letting her suffer countless daily attacks. Without a doubt, the roosters may kill her with this constant attention. However, know that if you do cull the hen, I would expect this behavior to transfer to another hen. You could pull her and see how the flock reacts, before making a final decision.
Wishing they would all be nice does not work. As the flock manager, you have to make some tough decisions. Once you do, you will be glad you did. This set up is causing a lot of tension in the whole flock, even the ones not being attacked.
Some tough decisions, but they are important decisions to make.
Mrs K